1. Field of the Invention
The present invention relates to a conductive copper paste used to form conductive coatings on substrates such as ceramics, insulators and the like.
2. Description of the Prior Art
In general, it is customary to use a conductive paste to form conductive coatings such as conductive patterns of printed circuit boards, electrodes of ceramic capacitors and the like. For example, printed circuit boards are manufactured by the steps of preparing ceramic green sheets, firing the ceramic green sheets to prepare sintered ceramic sheets, applying a conductive paste on the resultant sintered ceramic sheets by screen printing, and then firing the printed ceramic sheets to change the coated conductive paste into a conductive coating. Alternatively, printed circuit boards are manufactured by the steps of preparing ceramic green sheets, applying a conductive paste on the green sheet by screen printing or coating, and firing the coated green sheets to produce sintered ceramic sheets with baked conductive coatings.
The conductive paste used in such manufacturing steps consists of a copper powder dispersed in an organic vehicle to minimize warps or strains of the substrates which are caused by a difference in shrinkage factor between the conductive paste and the ceramic green sheet.
However, such a conductive paste has the following problems. For example, it requires firing or baking in a non-oxidizing atmosphere to prevent the copper from oxidizing. For this reason, former process requires the firing twice, thus making it difficult to improve the production efficiency of printed circuit boards. The latter process makes it possible to improve the efficiency of production by sintering the ceramic sheet simultaneously with the baking of the conductive paste coating on the ceramic green sheet. However, this process has a problem in that the conductive coating occasionally peels off from the substrate because of poor bond strength between the ceramic substrate and the baked conductive coatings. This problem also applies to the former process as the second firing must be carried out under the same conditions as that of the latter.
To solve such problems, the inventors have tried to incorporate glass frit of a lead borosilicate system into a conductive paste consisting of copper powder dispersed in an organic vehicle. However, it is considerably difficult to improve the bond strength between the ceramic substrate and the baked conductive coating as lead oxide contained in the glass frit is reduced to a metal during firing in the non-oxidizing atmosphere.